1. Field of the Invention
The present invention relates to a displacement detecting device for optical head for detecting focus displacement and track displacement of irradiation light with respect to an irradiated face of an information recording medium.
2. Description of the Related Art
An optical device formed by a combination of a convex lens and a cylindrical lens have been used for detecting displacement of focal positions of a laser beam from an optical head irradiating on a surface of an information recording medium as disclosed in JPB 53-39123 and JPB 57-12188, for example. The above mentioned optical device of the prior art cannot be produced in a sufficiently small size with a light weight. Then, a quadrifid Fresnel zone plate has been improved to be used in a displacement detecting device instead of the above optical devices.
FIG. 19 is an oblique view showing a conventional displacement detecting device for optical head which is described in JP-A-2-87336.
In FIG. 19, laser light emitted from a laser light source 1 is converted into a bundle of parallel rays by a collimator lens 2. The collimated rays are applied to an information recording medium 5 via a beam splitter 3. The irradiating laser rays are focused on the information recording medium 5 by an object lens 4. This object lens 4 is driven by a driver which is not illustrated and is subjected to feedback control so that rays may be always focused on the information recording medium 5. Reflected rays from the information recording medium 5 are splitted by the beam splitter 3 via the object lens 4 and led to a displacement detecting system called "astigmatism system". Rays for detecting focus displacement splitted by the beam splitter 3 are applied onto a quadrifid photodetector 9 via a quadrifid Fresnel zone plate 20. The quadrifid Fresnel zone plate 20 diffracts rays for detecting focus displacement to convert into an astigmatic bundle of rays having a wave front which is not homocentric.
The quadrifid Fresnel zone plate 20 used here is shown in FIGS. 20A and 20B. FIG. 20A is a top view of the plate. FIG. 20B is a sectional view of the plate seen along XXB--XXB. This quadrifid Fresnel zone plate 20 is divided into four sectors 20a, 20b, 20c and 20d by two partition lines 20l and 21l passing through the center 20o which are perpendicular to each other. Further, in opposing diagonal sectors 20a and 20c, and 20b and 20d, optical unevenness taking the shape of concentric circles as represented by shaded regions in FIG. 20A are formed so that primary diffracted rays may be converged at the same focal length.
The quadrifid Fresnel zone plate 20 and the quadrifid photodetector 9 thus formed are disposed so that the two partition lines 20l and 21l of the quadrifid Fresnel zone plate 20 may coincide with the two partition lines of the quadrifid photodetector 9 when they are viewed from the direction of the optical axis. Detected signals A, B, C and D respectively fed from the photodetecting sectors 9a, 9b, 9c and 9d of the quadrifid photodetector 9 are subjected to processing in adders 11 and 12 and a subtracter 13 to output a focus displacement signal F. In addition, the detected signals A, B, C and D are subjected to processing in a subtracter 15 to output a track displacement signal T. The focus displacement signal F and the track displacement signal T are represented by the following equations. EQU F=(A+C)-(B+D) EQU T=(D-B)
Depending on the position of the object lens 4 with respect to the information recording medium 5, images are formed on the quadrifid photodetector 9 as shown in FIGS. 21A, 21B, 22A, 22B, 23A and 23B. That is to say, if the information recording medium 5 is disposed at this side of the focal point of the object lens 4 as shown in FIG. 21A, the displacement detecting rays form images so that images may become large on photodetecting sectors 9a and 9c of the quadrifid photodetector 9 and images may become small on photodetecting sectors 9b and 9d as shown in FIG. 21B. Therefore, the focus displacement signal F becomes positive.
If the information recording medium 5 is accurately located on the focal point of the object lens 4, the rays for detecting displacement form a circular beam on the quadrifid photodetector 9 as shown in FIG. 22A. Therefore, detection signals A, B, C and D respectively of the photodetecting sectors 9a, 9b, 9c and 9d have the same magnitude as shown in FIG. 22B. Therefore, the focus displacement signal F becomes 0.
If the information recording medium 5 is located beyond the focal point of the object lens 4 as shown in FIG. 23A, the rays for detecting displacement form images so that images may become small on the photodetecting sectors 9a and 9c of the quadrifid photodetector 9 and images may become large on the photodetecting sectors 9b and 9d as shown in FIG. 23B. Therefore, the focus displacement signal F becomes negative.
In this way, the focus displacement signal F varies according to the position of the information recording medium 5 with respect to the object lens 4. By feeding back this focus displacement signal F to the driver for driving the object lens 4 in the focus direction, control is exercised so that the laser rays may form a focal point just on the information recording medium 5.
The case where only focus displacement is involved but track displacement is absent is shown in FIGS. 21A, 21B, 22A, 22B, 23A and 23B. If the focus displacement is absent and track displacement is present, however, an image as shown in FIG. 24 is formed on the quadrifid photodetector 9. That is to say, the shape of the image is identical with that in focus as shown in FIG. 22B. However, the brightness varies according to the track displacement. FIG. 24 shows the case where track displacement has occurred in the lateral direction. By performing feedback according to the track displacement, control is exercised so that the laser rays may get on a track of the information recording medium 5 accurately.
Positions of partition lines of the quadrifid photodetector 9 and the quadrifid Fresnel zone plate 20 need not be the positions shown in FIG. 19 as long as partition lines of the quadrifid photodetector 9, respectively, coincide with the corresponding partition lines of the quadrifid Fresnel zone plate 20 when viewed from the direction of the optical axis. For example, the quadrifid photodetector 9 and the quadrifid Fresnel zone plate 20 may be so disposed as to be rotated by 45 degrees around the optical axis as compared with the illustrated positions. In this case, the track displacement signal T is represented by the following equation. EQU T=(A-C)+(D-B)
In the conventional optical head as described above, the quadrifid Fresnel zone plate 20 is used to convert rays for detecting focus displacement into an astigmatic bundle of rays and form an image on the quadrifid photodetector 9. The quadrifid Fresnel zone plate 20 is fabricated by means of a precision machining technique used in the semiconductor manufacturing process. In this process, it is necessary to first draw a mask by using an electron beam direct lithography device or pattern generator, or the like.
However, the conventional quadrifid Fresnel zone plate 20 is shaped in curves. In order to draw the curves accurately, it was necessary to make the beam diameter small in the electron beam direct lithography device or the pattern generator, or the like, set and control a drawing position finely. For manufacturing such a mask having curves, therefore, enormous computation time and exposure time were needed, and the production cost became high, resulting in problems.